The present invention relates to a solder paste and more particularly to a lead-free, zinc-containing solder paste which comprises a powder of a zinc-containing, lead-free solder alloy in admixture with a soldering flux.
Snxe2x80x94Pb alloys have been used in soldering since ancient times, and they are still the most popular solders for soldering electronic components to printed circuit boards or other substrates.
Snxe2x80x94Pb alloys have a eutectic composition of approximately 63% Sn with a balance of Pb. This composition has a low melting temperature of 183xc2x0 C., thereby making it possible to perform soldering in a temperature range of from 220xc2x0 C. to 230xc2x0 C., in which range there is no substantial thermal damage to heat-sensitive electronic components. The eutectic Snxe2x80x94Pb alloy, called eutectic solder, has extremely good wettability and solderability, and since it does not have a difference between its liquidus and solidus temperatures (i.e., no solidification temperature range exists), solidification occurs instantaneously during soldering when the melting point is reached. As a result, even if vibrations or mechanical shocks are applied to parts to be soldered during soldering, cracking or detachment of the resulting soldered joints does not occur.
Discarded electronic appliances including televisions, radios, audio or video recorders, computers, copying or printing machines, etc. are generally disposed of in landfills, since such appliances are composed of various materials such as synthetic resins used for housings and printed circuit boards, and metals used for wires and other electric connections and frames, which are not suitable for disposal by incineration.
In recent years, the phenomenon of acid rain has become serious due to discharge of sulfur oxide into the atmosphere by extensive use of fossil fuels such as gasoline and fuel (heavy) oils. Acid rain penetrates into the ground and causes dissolution of the solders present in discarded electronic appliances buried in the landfills, thereby producing contamination of the groundwater with lead. If such contaminated groundwater is ingested by people for many years, the accumulation of lead in their bodies may result in lead poisoning (plumbism).
From this viewpoint, it has been desired in the electronics industry to use a lead-free solder alloy for soldering electronic components. Conventional lead-free solder alloys are Sn-based alloys such as Snxe2x80x94Ag, Snxe2x80x94Sb, Snxe2x80x94Bi, and Snxe2x80x94Zn alloys.
Snxe2x80x94Ag alloys form a eutectic composition of Sn-3.5Ag, but the melting temperature, i.e., eutectic temperature of this composition is relatively high (221xc2x0 C.). Even if this eutectic composition having the lowest melting temperature among Snxe2x80x94Ag alloys is used as a solder alloy, the soldering temperature will be as high as from 260xc2x0 C. to 270xc2x0 C., which may cause thermal damage to heat-sensitive electronic components during soldering, thereby deteriorating or even destroying their functions.
Of Snxe2x80x94Sb alloys, an Sn-5Sb alloy has the lowest melting temperature, but its melting temperature is as high as 235xc2x0 C. on the solidus line and 240xc2x0 C. on the liquidus line. Therefore, its soldering temperature is in the range of from 280xc2x0 C. to 300xc2x0 C., which is still higher than that of an Sn-3.5Ag alloy, and thermal damage to heat-sensitive electronic devices cannot be avoided.
Snxe2x80x94Bi alloys have a eutectic composition of 42% Snxe2x80x94Bi with a melting temperature of 139xc2x0 C., which is considerably lower than that of the above-described conventional Snxe2x80x94Pb eutectic solder (183xc2x0 C.). Therefore, Snxe2x80x94Bi alloys may be considered to be potentially usable as lead-free solders from the viewpoint of melting temperatures. However, Snxe2x80x94Bi alloys are too brittle and hard to meet the mechanical properties such as tensile strength and elongation that are required for solder alloys.
Snxe2x80x94Zn alloys have a eutectic composition of Sn-9% Zn with a melting temperature of 199xc2x0 C. This eutectic composition is advantageous in that its melting temperature is close to that of conventional Snxe2x80x94Pb eutectic solder (183xc2x0 C.). Another advantage of Snxe2x80x94Zn alloys is that their mechanical properties are superior to those of Snxe2x80x94Pb alloys. However, Snxe2x80x94Zn alloys have poor solderability.
In order to improve the solderability of Snxe2x80x94Zn alloys and further enhance their mechanical properties, a number of solder alloys based on an Snxe2x80x94Zn alloy and containing one or more additional elements such as Ag, Cu, Bi, In, Ni, and P have been proposed.
With these improved Snxe2x80x94Zn based solder alloys containing one or more additional elements, a considerably satisfactory solderability can be achieved as long as these alloys are used in the form of wire solder for soldering with a soldering iron along with an appropriate soldering flux. However, when these Snxe2x80x94Zn based solder alloys are used in the form of a solder paste, which is a mixture of a powder of such a solder alloy and a soldering flux in a viscous fluid, they do not work successfully or not exhibit satisfactory solderability. Thus, a solder paste formed from an Snxe2x80x94Zn based solder alloy may cause non-wetting or dewetting during soldering whereby the areas of a substrate to be soldered are not wetted by the solder completely and have solder balls thereon. Even though the solder after soldering appears by visual observation to be sound or wet the areas completely, it may contain internal pit-like voids at the interface between the solder and the substrate, as can be seen when the solder is peeled off.
The solderability of a solder paste formed from an Snxe2x80x94Zn based solder alloy can be improved by using an activated flux containing a strong activator which can effectively enhance the spreading of the molten solder alloy. However, the strong activator can react with zinc (Zn) present in the solder alloy to oxidize or corrode it in a short period of time and cause the solder to lose its metallic nature, resulting in a significant deterioration in solderability. Thus, a solder paste formed from an Snxe2x80x94Zn based solder alloy or other Zn-containing solder alloy (such a solder paste being hereinafter referred to as xe2x80x9cZn-containing solder paste) generally suffers the problem of detrimental alterations, i.e. aging, after storage for a relatively short period.
The detrimental alterations with time (hereinafter referred to as aging) of a Zn-containing solder paste appear as a change in viscosity. Thus, a Zn-containing solder paste immediately after it is prepared has an appropriate viscosity which makes it easy to stir with a spatula or stirring rod and which is suitable for application by screen printing or feeding with a dispenser. However, after it is stored for a certain period of time on the order of one or two weeks, it has an increased viscosity due to aging and is difficult to stir.
When such an aged solder paste having an increased viscosity is applied to a printed circuit board by screen printing or with a dispenser and then heated in a reflow furnace, the solder may not entirely melt or a large quantity of oxides formed in the solder alloy may cause the formation of solder balls as the solder melts. Even with a freshly prepared Zn-containing solder paste, if reflow soldering is conducted in an oxygen-containing atmosphere such as air, the molten solder may not spread adequately and good solderability may not be obtained. Therefore, reflow soldering must be performed in an inert gas atmosphere, thereby adding to operating costs.
It is an object of the present invention to provide a Zn-containing solder paste which is less susceptible to aging and has an extended shelf life.
It is another object of the invention to provide a Zn-containing solder paste which exhibits satisfactory solderability when reflow soldering is performed in air.
It has been found that the addition of a glycidyl ether compound to a flux used to prepare a Zn-containing solder paste has an effect of stabilizing the resulting solder paste against aging and improving the solderability of the solder paste, although the mechanism for this effect has not been clearly elucidated.
Such a favorable effect of the glycidyl ether compound on stability and solderability of a solder paste is prominent particularly with Zn-containing solder pastes, but it can be achieved to some extent with other solder pastes. Thus, the addition of a glycidyl ether compound to a flux is generally effective in all kinds of solder pastes in order to retard aging and improve solderability.
According to one aspect of the present invention, a solder paste comprises a powder of a solder alloy, preferably of a lead-free solder alloy, in admixture with a soldering flux with a glycidyl ether compound being added to the soldering flux.
More particularly, the present invention provides a lead-free solder paste comprising a powder of a zinc-containing, lead-free solder alloy in admixture with a soldering flux (namely, a Zn-containing solder paste) wherein a glycidyl ether compound is added to the soldering flux, preferably in an amount of from 0.1% to 5.0% by weight of the flux.
The glycidyl ether compound is a compound having at least one glycidyl ether moiety 
in the molecule.
In a preferred embodiment, the Zn-containing solder alloy is an Snxe2x80x94Zn based alloy including an Snxe2x80x94Zn alloy, and more preferably an Snxe2x80x94Zn-Bi alloy.
In accordance with the present invention, a Zn-containing solder paste, which has been considered to have a shelf life of approximately one week when stored at 25xc2x0 C. or below, can be stored for 4 weeks or longer without a significant aging, thereby facilitating the practical use of Zn-containing solder pastes.